The present invention relates to a technology for controlling the shifts of an automatic transmission.
A vehicular automatic transmission generally uses a plurality of planetary gear sets, each of which is composed of revolving components such as a sun gear, a ring gear and a carrier. The automatic transmission is constructed by connecting predetermined ones of the revolving components, by connecting any of the revolving component selectively to an input shaft through clutch means, by fixing another revolving component selectively by brake means, and by connecting an output shaft to still another revolving component. In the automatic transmission thus constructed, the number of speed stages to be set, the gear ratios at the individual speed stages, and the number of revolution, loads or torques of the revolving components are varied in dependence upon the number of the planetary gear sets used, the manner how to connect the individual revolving components, and the number of and the manner how to dispose frictional engagement means such as the clutch means or the brake means, so that a remarkable variety of automatic transmissions can be constructed. All the structures could not be put into practical use, although they could be made in principle. It is not easy in the least to create a structure which can sufficiently meet the practical requirements such as feasible production, small size and low weight, ample shift controllability and excellent durability. For example, even an automatic transmission using three planetary gear sets to set five forward five and one reverse speed stages might have an excessively large gear ratio for the reverse stage in dependence upon the arrangement of the frictional engagement means such as the clutches and the gear ratio (i.e., the ratio of tooth numbers of the sun gears and the ring gears). For this automatic transmission, it would be necessary to increase the capacities of the frictional engagemetn means, thus raising a problem that the durabilities of the frictional engagement means and the bearings might be deteriorated. In order to solve this problem, as disclosed in Japanese Patent Laid-Open No. 60-57036, there is proposed an automatic transmission which is constructed to have a reverse gear ratio approximate to that for the forward 1st speed.
The structure of the automatic transmission according to this proposal will be briefly described in the following. This automatic transmission is composed mainly of three single pinion type planetary gear sets and is constructed by connecting the sun gear of the first planetary gear set and the sun gear of the second planetary gear set always or selectively, by connecting the sun gear of the first planetary gear set and the carrier of the second planetary gear set, if necessary, through a clutch, by connecting the ring gear of the second planetary gear set and the ring gear of the third planetary gear set, by connecting the carrier of the first planetary gear set with those ring gears, and by connecting the carrier of the second planetary gear set and the sun gear of the third planetary gear set through a clutch. The input shaft is connected through a clutch to the sun gears of the first and second planetary gear sets, which are connected to each other, and further to the ring gear of the first planetary gear set through another clutch. On the other hand, the output shaft is connected to the carrier of the third planetary gear set. As the brake means for stopping the revolutions, moreover, there are provided a brake for fixing the sun gear of the first planetary gear set and the sun gear of the second planetary gear set and a brake for fixing the sun gear of the third planetary gear set.
In Japanese Patent Laid-Open No. 60-57036, there is disclosed a structure in which a one-way clutch is arranged in parallel with the aforementioned clutches and brakes so as to facilitate the shift controls. In the automatic transmission disclosed in Japanese Patent Laid-Open No. 60-57036, moreover, the absolute values of the gear ratios for the forward 1st speed and reverse stages are so approximate that the automatic transmission can be small-sized while preventing any excessive torque from being applied to the revolving components at the speed stages of low frequencies of use.
Incidentally, it is preferable that the automatic transmission be as small and light as possible so as to have its mountability and fuel consumption improved as is well known in the art. For this purpose, it is desirable to reduce the sizes and number of the components. In the automatic transmission disclosed in Japanese Patnet Laid-Open No. 60-57036, however, the parts are small-sized by avoiding establishment of excessive torque, and a one-way clutch is provided for facilitating the shift controls. Since, however, each speed stage has only one kind of engagement/release pattern for the clutches and brakes to set that speed stage, it could be deduced that only one shift is provided for causing each one-way clutch to function its automatic changing characteristics. In order to provide an excellent controllability for any of the shifts, therefore, the automatic transmission is required to have additional one-way clutches. In short, the existing automatic transmission thus far described is disadvantageous in that its size is enlarged by the number of its necessary parts increased for improving the shift controllability. If the number of one-way clutches is reduced to avoid the disadvantage, the shift shocks might probably be increased to raise another problem that the shifts would become difficult to control.